Process for the electrolytic deposition of copper



PROCESS FOR ELECTROLYTIC DEPOSITION OF COPPER 9 Claims. (Cl. 204-42) Theaqueous copper ba'ths used so far can be distinguished in two groups,the acid and the alkaline copper baths. Of the acid copper baths thedisadvantage is known that the deposition of contact copper on profiledobjects cannot or only with difiiculty be prevented, whereas thealkaline copper baths always contain the copper complex bound to thecyanide ion. The disadvantages of cyanidic copper baths are evident;they are toxic, give off toxic vapors and are unstable, i.e. the cyanidedecomposes while forming substances which collect in thebaths, whilstmoreover CO is taken up from the air. When the cyanide bath has becomeuseless, its removal forms a problem per se.

The process according to the present "invention relates to anelectrolytic deposition of copper fromaqueousco'pper salt solutions inthe presence of a diamine. For the sake of completeness it is remarkedthat copper baths containing a diamine are known per's'e fr'omTransactions Electrochemical Society 71 (1937), 255-261;73 (1938),371376, and 78 (1940) 303-307. Accordin'g to the invention a diamine isused, the boiling point of which at atmospheric pressure lies betweenapproximately 100 and 150 C., in an amount of at least 1 :molper liter.The thus prepared baths do not have the disadvantagesof the acid andalkaline copper baths; the copper deposits are very resistant and freeoftpores,'whilst the metal distribution of the bath is good. Thedepositalso has a good tarnish resistance. The concentration of thebaths prepared according to the invention can be chosen high, namely upto approximately 250 g. of copper sulfateper liter, so that quickoperation is possible. With a cathode density of 5 to 7 a./dm. more than11 micron of copper is deposited per minute.

It has been observed that the diamine copper complex formed obtains itsgreatest solubility when per mol of copper salt approximately 2 mols ofdiamine are added.

In the preferred amine concentration a part of the amine gets lostrelatively quickly by evaporation. Such losses are expensive, theevaporated amines moreover are obnoxious to health and finally one doesnot know the concentration of the amine in the bath at any moment.

Furthermore it has now been found that the disadvantage of thevolatility of the amine used can be obviated by adding to the bath alsoa monoamino carboxylic acid the complexity constant of which is higherthan that of the diamine used. These baths have all advantages adheringto the use of baths with a high diamine concentration, but they yieldmoreover a deposit which can very easily be polished, which means aconsiderable saving of expenses in the mechanical treatment.

In Table I some diamines are indicated the boiling point of which liesbetween 100 and 150 C., together with their boiling points and thenegative logarithm of the complexity constant, pK

2,887,442 Patented May 19, 1959 2 in'which K (Cu"+).(En) (Cu'Enl HereinEnre'presents the concentration Ofthe diamine, e.g. of the ethylenediamine, (Cu represents the concentration of the free cupric .ions and(CuEn represents the. concentration of the cupric ion bound to thediamine.

In Table II some amino acids are mentioned together with the negativelogarithm of the complexity constant of the association reaction withthe free cupric ion, 'in which Herein AZ represents the amino acid andCuAZ the cupric amino acid complex.

TABLE II P a-Alanine =a-c -c c c c. 15.0 fl-Alanine 12.8 Glycine 15.4Glutamic acid l 15.0

'y-Aminobutyric acid 8.6

Preferably amino acids are usedthecomplexity constant of which is atleast some tenth powers greater than that of the diamine with which itis combined, in other words the pK of which is at least 2 smaller thanthe pK of the diamine used. In that case namely the greater part of thecupric ion isbound to adiamine complex and for a much smaller part (withadilference of2 approximately one-thousandth of the above-mentionedpart) asan amino acid complex. The amino acid complexes namely, ingeneral, are less soluble in the usedsolutions than the diamine'complexes,sotlrat in the reversed case insoluble amino acid complexesmay be formed.

An advantage of the copper baths prepared according to the invention isthe following. When a small excess of the diamine (about 2.2 mols) permol of copper salt is used, the copper as well as the greater part ofthe diamine are bound to a complex of slight volatility, evaporation ofthe excess of diamine, however, still being possible. This evaporationis controlled according to the invention in the copper bath by replacingthe previously necessary excess of diamine by a non-volatile amino acidwhich binds only a low percentage of the copper and is rather wellsoluble itself.

As appears from Table II monoamino monocarboxylic acids as well asmonoamino dicarboxylic acids, such as glutarnic acid can be used. Ifdesired, a wetting agent may be added to the bath. This wetting agentmay be of the usual nature, such as e.g. an alkaline salt of a benzenesulfonic acid or lauryl sulfonic acid, but a deposit with improvedproperties, especially in respect of the brightness is obtained when thesulfate of a primary aliphatic alcohol with a branched carbon chain with6 to 16 carbon atoms is added to the bath as the wetting agent.

The invention is elucidated by way of the following examples: Example 1In the thus prepared bath brushed iron objects were copper plated at atemperature of 58 C., a cathode density of 8 a./dm. and an anode densityof 1.2 a./dm. The pH value of the bath was about 8.7. The cathode wascontinuously kept moving as usual.

After 30 minutes a copper layer with an average layer thickness of about50 microns had deposited: 94 microns on the projecting portions of theobject and 34 microns in the hollow portions. This points to anextremely uniform distribution of the metal. The deposit everywhere hadthe same high brightness and could immediately be nickel-plated withoutafterpolishing.

Example 2 A copper bath was prepared with the following composition:

Copper sulfate 5 aq 225 g./l. (0.9 mol). Propylene diamine-1.2 148 g./l.(2.0 mols).

Glutamic acid 66 g./l. (0.45 mol). 2.butyl-decanol sulfate 0.8 g./1.

With the aid of the thus prepared bath brushed iron objects were copperplated at 56 C. The cathode denwhich comprises electrolyzing an aqueoussolution containing an acidic copper salt, at least one mol per liter ofa diamine with a boiling point between about 100 C. and 150 C. atatmospheric pressure and a saturated aliphatic monoamino carboxylic acidsuflicient in amount to complex the copper, the decomposition constantof the complex being greater than that of the complex of the copper withthe diamine used.

2. The process of. claim 1, wherein the decomposition constant of thecomplex of the acid is at least times as great as that of the complex ofthe copper with the diamine used.

3. The process of claim 2, wherein a sulfate of a primary aliphaticalcohol with a branched carbon chain having from 6 to 16 atoms is addedto the solution as a wetting agent.

4. A process for the electrolytic deposition of copper, which compriseselectrolyzing an aqueous solution containing an acidic copper salt, atleast one mol per liter of a diamine with a boiling point between about100 C. and C. at atmospheric pressure and a monoamino carboxylic acidselected from the group consisting of rat-alanine, B-alanine, glycine,glutamic acid and 'yaminobutyric acid, the amount of the acid beingsufiicient to complex the copper.

5. An aqueous electrolyte for the electrolytic depositionof copperconsisting essentially of an aqueous solution of an acidic copper salt,at least one mol per liter of a diamine with a boiling point betweenabout 100 C. and 150 C. at atmospheric pressure and a saturatedaliphatic monoamino carboxylic acid sufiicient in amount complex beinggreater than that of the complex of the to complex the copper, thedecomposition constant of the copper with the diamine used.

6. An aqueous electrolyte for the electrolytic deposition of copperconsisting essentially of an aqueous solution of an acidic copper salt,at least one mol per liter of a diamine with a boiling point betweenabout 100 C. and 150 C. at atmospheric pressure and a monoaminocarboxylic acid selected from the group consisting of a-alanine,fl-alanine, glycine, glutamic acid and 'y-aminobutyric acid, the amountof the acid being sufficient to complex the copper.

7. The electrolyte of claim 6, comprising about one half mol per literof the monoamino carboxylic acid.

8. The process of claim 4, wherein the molar proportion of the acid is0.1 to 0.6 times the molar proportion of copper.

9. The electrolyte of claim 6, wherein the molar proportion of the acidis 0.1 to 0.6 times the molar proportion of copper.

References Cited in the file of this patent 'UNITED STATES PATENTSGreenspan Apr. 2, 1940 Harford Aug. 8, 1944

1. A PROCESS FOR THE ELECTROLYTIC DEPOSITION OF COPPER, WHICH COMPRISESELECTROLYZING AN AQUEOUS SOLUTION CONTAINING AN ACIDE COPPER SALT, ATLEAST ONE MOL PER LITER OF A DIAMINE WITH A BOILING POINT BETWEEN ABOUT100* C. AND 150* C. AT ATMOSPHERIC PRESSURE AND A SATURATED ALIPHATICMONOAMINO CARBOXYLIC ACID SUFFICIENT IN AMOUNT TO COMPLEX THE COPPER,THE DECOMPOSITION CONSTANT OF THE COMPLEX BEING GREATER THAN THAT OF THECOMPLEX OF THE COPPER WITH THE DIAMINE USED.